1. Field of the Invention
This invention relates to a process for the separation of dicarboxylic acids by membrane filtration.
2. Discussion of Related Art
Conventional processes for separating mixtures of dicarboxylic acids include crystallization, rectification and extraction. However, narrow limits are frequently imposed on the application of these processes on an industrial scale since the dicarboxylic acids to be separated differ only slightly in regard to the physical properties of relevance to these processes. In cases such as these, separation can often only be carried out incompletely and at considerable cost. This is always the case when relatively large carboxylic acid molecules only differ from one another in regard to a functional group or another comparable molecular parameter.
German Patent Publications Nos. 21 40 133 and 28 53 847 describe processes for the production of dicarboxylic acids by fermentation and for the purification thereof. The mixture of dicarboxylic acids which are formed are separated by chromatography in the form of their methyl esters solely for analytical characterization. A similar fermentative process is described in German Patent Publication No. 21 64 626. The mixtures of hydroxy, keto and dicarboxylic acids formed are separated by distillation of the alkyl esters under reduced pressure.
German Patent Publication No. 29 51 177 describes the purification of dicarboxylic acids produced by fermentation by recrystallization from a solvent. Fractional crystallization is another known method for the separation of dicarboxylic acid mixtures.
Membrane filtration is generally used for the separation of substances according to gram molecular weight.
On the other hand, it is known from Process Biochemistry, March/April 1983, pages 8-12, that the permeability of membranes to certain dissolved substances can be controlled through the choice of the pH value of the solution to be filtered. For example, the concentration of acetic acid or lactic acid in aqueous solution can be concentrated by means of a special membrane. However, there is no suggestion in this literature reference of a process for the specific separation of carboxylic acids and special dicarboxylic acids.
In the main, the above-mentioned processes for the separation of mixtures of dicarboxylic acids differing only slightly in their chemical structure are only suitable for laboratory quantities.
Accordingly, an object of the present invention is to provide a separation process for dicarboxylic acids which combines a high separation efficiency and increased capacity such that the process is suitable for use on a relatively large commercial scale.